Start it in safe mode. F8 at the splash screen.
You'll see the Admin account for certain.
Then go to accounts/users in Control panel.
See if you can set it up again.
Or just try the Users/accounts in Control Panel in a normal boot.
See what the settings are.
Settings may have been changed.
No Windows OS's are really secure.
XP certainly isn't, regardless of MS's protestations.

Rusty.
Btw: Welcome to TIMO.